In a wireless communication system, endeavors for improving the drain efficiency of the system are being made in various forms. Particularly, a way to decrease the power consumption of a power amplifier that operates within the wireless communication system is emerging as a typical way among various ways for improving the drain efficiency of the wireless communication system. Also, to decrease the power consumption of the power amplifier operating within the wireless communication system, elements included in the power amplifier can be diversified. For example, a power amplifier using a Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistor can be used to improve the drain efficiency of the wireless communication system. Also, a Doherty amplifier applying a GaN (gallium nitride) transistor having an excellent drain efficiency compared to the LDMOS transistor can be used to improve the drain efficiency of the wireless communication system.
However, the power amplifier used to improve the drain efficiency of the wireless communication system has nonlinearity commonly. In other words, the ideal power amplifier determines output power in proportion to input power, but an actual power amplifier cannot keep this linearity if the input power exceeds a specific level. As a result, a power gain compared to the output power is not kept constant, and the power gain decreases as the output power increases. If the linearity of the power amplifier is not kept like this, a problem of deteriorating the performance of the wireless communication system takes place.